FIG. 1 shows a cross-sectional side view of a torque converter 10 including a conventional hub assembly 12. Hub assembly 12 includes a hub 14 fixed to a turbine shell 16. Hub 14 includes two seals 18, 20 on the outer radial surface 22 thereof for contacting a further hub 24, which is fixed to a front cover 26 of torque converter 10 and is rotatable independently of hub 14 when piston 28 is not clamping clutch plate 30 against front cover 26 to engage lock-up clutch 32. Hub 14 includes a channel 34 supplying fluid to a region 36 formed radially between hubs 14, 24 and sealed axially by seals 18, 20.
A lock-up pressure fluid is applied through the center of a transmission input shaft into hub 14. The fluid then crosses a back pressure chamber flow path before reaching piston 28. The back pressure flow path is isolated from both a torque converter charge pressure and an apply pressure by seals 18, 20 between hubs 14, 24. The flow paths in these hubs cause a delay and pressure loss/bump as piston 28 is being engaged. When piston 28 is engaged, the apply chamber pressure increases, causing a pressure difference between the apply chamber and the back pressure chamber applying clutch 32 with a force relative to the differential pressure. As piston 28 moves towards the back pressure chamber, the back pressure chamber reduces in size and attempts to evacuate fluid without a buildup of pressure to ensure maximum controllability of differential pressure across the piston for increased clutch controllability. Flow is choked in the cross over interface causing back pressure in piston 28 and delayed clutch response time.